1. Field of the Invention
The present invention relates to a thin-film vapor deposition apparatus, and more particularly to a thin-film vapor deposition apparatus suitable for depositing in a vapor or gas phase a thin film, having a high dielectric constant, of barium titanate, strontium titanate, or the like.
2. Description of the Prior Art
Recent years have shown a need for ever-increasing levels of integration of semiconductor integrated circuits produced by semiconductor manufacturers. Research and development activities are switching from efforts to manufacture present DRAMs (dynamic random-access memories) having a storage capacity of megabits toward attempts to produce future DRAMs having a storage capacity of gigabits. Such DRAMs contain capacitive elements which should preferably provide as large a capacity as possible while taking up as small an area as possible. One dielectric thin film which is presently used in the art to provide such a capacitive element comprises a film of silicon oxide or a film of silicon nitride. These films have a dielectric constant of 10 or smaller. Among promising thin-film materials proposed for use in the future are metal oxides including tantalum pentoxide (Ta.sub.2 O.sub.5) having a dielectric constant of about 20, barium titanate (BaTiO.sub.3) having a dielectric constant of about 300, strontium titanate (SrTiO.sub.3), or a mixture thereof, i.e., barium strontium titanate.
One conventional vapor deposition apparatus for fabricating such a thin film of metal oxide is disclosed in Japanese laid-open patent publication No. 63-307276, for example. Such disclosed vapor deposition apparatus includes a reaction tube for accommodating a substrate that is kept in a heated condition. A gas composed of a plurality of organic metal compounds is introduced into the reaction tube to deposit a metal compound generated in a vapor-phase reaction onto the heated substrate. The disclosed vapor deposition apparatus has a heating device for heating the wall surface of a gas supply system for introducing the gas composed of organic metal compounds, a heating device for heating the reaction tube, a supply device for supplying a gas containing oxygen, an inlet tube for introducing the gas containing oxygen closely to the substrate, and an inlet tube for introducing the gas from the gas supply system into the reaction tube.
Generally, the gas composed of organic metal compounds is highly unstable in oxygen. Therefore, if oxygen were introduced into the reaction system, it would tend to cause an explosion or develop a premature reaction in regions other than the substrate. To avoid such an explosion or a premature reaction, both the inlet tubes extend separately to a position near the substrate to introduce and mix the oxygen-containing gas and the organic metal compound gas quickly and uniformly with each other closely to the substrate for thereby growing a thin film of metal oxide on the substrate. Since the organic metal compound gas is in a liquid phase at normal temperature, the inlet tube for introducing the organic metal compound gas and the entire reaction system have to be heated by the heating devices to prevent the organic metal compound gas from being condensed. The substrate is placed on a susceptor which is associated with a heater for heating the susceptor to and keeping it at a temperature suitable for a reaction for vapor-phase deposition.
Japanese laid-open patent publication No. 5-335248 shows a thin-film vapor deposition apparatus having a heater for heating a semiconductor substrate placed in a reaction casing to effect vapor-phase deposition on the semiconductor substrate, and another heater for heating the inner wall of an inlet tube, which introduces a material gas into the reaction casing, so that the material gas will not be condensed in the inlet tube.
Japanese laid-open patent publication No. 4-364024 shows a thin-film vapor deposition method which uses an organic metal compound gas as a material gas. The disclosed method is aimed at the fabrication of an epitaxial growth layer of uniform thickness with good reproducibility from such an organic metal compound gas. Specifically, during the vapor-phase deposition process, the organic metal compound gas flows around a baffle plate disposed parallel to a substrate on which to deposit a thin film, and is delivered radially inwardly along a gas ejector disk through radial slots defined therein whose widths are not equal to each other. The gas flow delivered radially inwardly along the gas ejector disk is supplied onto the substrate which is being heated and rotated for thereby compensating for irregularities of the deposition rate. As a result, an epitaxial growth layer of uniform thickness can be produced with good reproducibility on the substrate.
For depositing a thin film of a metal oxide such as barium titanate or the like in a vapor phase, it is necessary that the reaction casing and the substrate be kept at totally different temperatures. For example, the reaction casing should be kept at a temperature ranging from 250.degree. C. to 260.degree. C., which is a lower limit temperature to prevent the thin-film material from being condensed and an upper limit temperature to keep the thin-film material gas in a vapor gas without being decomposed, with a temperature accuracy of .+-.2%, and the substrate should be kept at a temperature ranging from 400.degree. C. to 700.degree. C. with a temperature accuracy of .+-.1%. The thin-film vapor deposition apparatuses disclosed in the above publications fail to control the temperatures of the reaction casing and the substrate independently of each other. The heating device for heating the reaction tube typically comprises an electric oven or a high-temperature ribbon heater. Consequently, it is difficult to keep the inner wall of the reaction casing at a desired temperature in a region where the material gas is introduced, on account of the radiant heat applied from the heating device for heating the substrate.
A shower head for discharging a material gas to grow a thin film on a substrate is required to eject a flow of the material gas, which has been uniformly heated to a high constant temperature, with a uniform density across the entire surface of the shower head.
With the thin-film vapor deposition apparatus disclosed in the above publications, it is difficult to eject a flow of the uniformly heated high-temperature material gas with a uniform density across the entire surface of the shower head. For example, a thin-film vapor deposition apparatus disclosed in Japanese laid-open patent publication No. 4-364024 has a high-frequency coil for heating a susceptor from around a reaction casing, and a gas inlet manifold such as a gas ejector disk which is heated by radiant heat produced by the high-frequency coil. Since the material gas is ejected from the slots defined in the gas inlet manifold and having widths which are not equal to each other, it is difficult to supply a flow of the uniformly heated high-temperature material gas with a uniform density to the substrate.